Method of preparing a titanium and zirconium-based oxide, the oxides thus obtained and the use of same as catalysts

ABSTRACT

The invention concerns a process for preparing an oxide based on zirconium and titanium in which a liquid medium containing a zirconium compound and a titanium compound is formed; said medium is then heated; the precipitate obtained from the end of the preceding step is recovered and optionally, said precipitate is calcined. The invention also concerns an oxide based on zirconium and titanium. Said oxide can comprise in the range 30% to 40% by weight of titanium oxide and in this case it has a pure ZrTiO 4  type structure or a mixture of phases of structure type ZrTiO 4  and structure type anatase. Said oxide can also comprise in the range 10% to 20% by weight of titanium oxide and it then has a specific surface area of at least 40 m 2 /g after calcining for 5 hours at 800° C.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR02/04288 filed on Dec. 11, 2002.

The present invention relates to a process for preparing an oxide basedon zirconium and titanium, to oxides based on zirconium and titaniumobtained thereby, and to the use of said oxides as catalysts.

It is known that oxides based on zirconium and titanium can be used as acatalyst or catalyst support. However, known processes for preparingthem result in products with an unstable specific surface area. This issubstantially reduced when the products are subjected to hightemperatures, for example over 600° C., and they can reach values whichrender those products unsuitable for high temperature use as a catalystsupport.

One aim of the invention is to develop a process which produces oxidesbased on zirconium and titanium with a specific surface area thatremains high even after calcining at high temperatures.

To this end, the process of the invention for preparing an oxide basedon zirconium and titanium is characterized in that it comprises thefollowing steps:

-   -   forming a liquid medium containing a zirconium compound and a        titanium compound;    -   heating said medium;    -   recovering the precipitate obtained at the end of the preceding        step;    -   optionally, calcining said precipitate.

Further, the invention concerns, as novel products and in accordancewith a first implementation, an oxide based on zirconium and titaniumwhich is characterized in that it comprises 30% to 40% by weight of anoxide of titanium and in that it has a ZrTiO₄ type structure, pure or asa mixture of phases with structure type ZrTiO₄ and with an anatase typestructure.

In a second implementation, the invention also concerns an oxide basedon zirconium and titanium which is characterized in that it comprises10% to 20% by weight of titanium oxide and in that it has a specificsurface area of at least 40 m²/g after calcining for 5 hours at 800° C.

Further characteristics, details and advantages of the invention willbecome clearer from the following description and non limiting exampleswhich are given by way of illustration.

It should be noted that throughout the description, the term “specificsurface area” means the BET specific surface area determined by nitrogenadsorption in accordance with the ASTM D 3663-78 standard establishedfrom the BRUNAUER-EMMETT-TELLER method described in the periodical “TheJournal of the American Chemical Society”, 60, 309 (1938).

The first step of the process of the invention consists of preparing amixture of a zirconium compound and a titanium compound in a liquidmedium.

The liquid medium is generally water.

The compounds are preferably soluble compounds. In particular, they canbe salts of zirconium and of titanium.

The mixture can equally be obtained either from compounds initially inthe solid state which are subsequently introduced into a stock, forexample water, or directly from solutions of said compounds andsubsequent mixing, in any order, of said solutions.

The zirconium compounds can be selected from zirconyl sulphate, zirconylnitrate, zirconium nitrate or zirconyl chloride. Zirconyl nitrate isparticularly suitable.

More particularly, the titanium compound can be titanium oxychloride.Titanyl sulphate, titanium nitrate and titanium tetrachloride can alsobe mentioned.

Once the initial mixture has been obtained, it is heated in accordancewith the second step of the process of the invention.

The temperature at which said heat treatment is carried out, also knownas thermohydrolysis, is preferably over 100° C. It can thus be in therange 100° C. to the critical temperature of the reaction medium, inparticular between 100° C. and 350° C., and preferably between 100° C.and 200° C.

The heating operation can be carried out by introducing the liquidmedium containing said species into a closed vessel (closed autoclavetype reactor), the necessary pressure then resulting only from heatingthe reaction medium (autogenous pressure). Under the temperatureconditions given above, and in aqueous media, it can be stated by way ofillustration that the pressure in the closed reactor can be between avalue of over 1 bar (10⁵ Pa) and 165 bars (1.65×10⁷ Pa), preferablybetween 3 bars (5×10⁵ Pa) and 10 bars (1.65×10⁷ Pa). Clearly, it i alsopossible to exert an external pressure which then adds to thatconsequent upon heating.

Heating can also be carried out in an open reactor for temperaturesclose to 100° C.

Heating can be carried out either in air or in an atmosphere of an inertgas, preferably nitrogen.

The treatment period is not critical and can vary within wide limits,for example between 1 and 48 hours, preferably between 2 and 24 hours.Similarly, the rate of temperature rise is not critical, and the fixedreaction temperature can be reached by heating the medium for between 30minutes and 4 hours, for example, these values being given by way ofindication alone.

At the end of this second step and in a particular variation of theinvention, it is possible to bring the reaction medium obtained to abasic pH. This variation can result in products with greater chemicalpurity.

This operation is carried out by adding to the medium a base such as anammonia solution, for example.

The term “basic pH” means a pH of more than 7 and preferably more than8.

After the heating step, a solid precipitate is recovered which can beseparated from its medium using any conventional technique forsolid-liquid separation, such as filtration, decanting, draining orcentrifuging.

The recovered product can then be washed using water or, optionally, abasic solution, for example an ammonia solution. To eliminate residualwater, the washed product can finally and optionally be dried, forexample in an oven or by spray drying, at a temperature which can bebetween 80° C. and 300° C., preferably between 100° C. and 200° C.

In a particular implementation of the invention, a specific type ofwashing can be employed, consisting of taking up the separated productin suspension in water and adjusting the pH of that suspension to avalue of more than 7 and preferably more than 8 by adding a base, forexample. After separating again, the product can be recovered for thesubsequent step or for a further wash of the same type.

In a final step of the process of the invention, after any washingand/or drying, the recovered precipitate can then be calcined ifnecessary. Said calcining can develop the crystallinity of the productformed and produce it essentially in the oxide form, and it can also beadjusted and/or separated as a function of the subsequent servicetemperature of the oxide of the invention, taking into consideration thefact that the higher the calcining temperature, the lower the specificsurface area of the product. Calcining is generally carried out in airbut clearly, calcining carried out in an inert gas is not excluded.

In a particular variation of the invention, calcining can be carried outin a confined atmosphere. This variation can result in products with ahigher surface area.

In practice, the calcining temperature is generally limited to aninterval in the range 300° C. to 1000° C.

The invention also concerns certain oxides based on zirconium andtitanium which will now be described.

In the present description, the proportions of oxides are given as theweight of titanium oxide over the composition as a whole, i.e.TiO₂/(ZrO₂+TiO₂).

The products in accordance with the first implementation of theinvention comprise in the range 30% to 40% by weight of titanium oxide.They can also have a pure ZrTiO₄ type structure. The term “pure” meansthat X ray diffraction analysis of the product does not reveal anystructure other than the ZrTiO₄ structure. This structure corresponds toreference 34-415 JCPDS.

The products can also be in the form of a mixture of phases of structuretype ZrTiO₄ and of structure type anatase TiO₂. However, structure typeZrTiO₄ is in the majority. The anatase TiO₂ structure type is shiftedvery substantially towards large interplanar spacings, probably due toinsertion of Zr⁴⁺ ions into the anatase structure. The presence of twostructures corresponds to products with the highest titanium oxidecontents, in particular to a content of 40%.

Finally, the products in this first variation have a specific surfacearea of at least 30 m²/g after calcining for 5 hours at 800° C. Moreparticularly, this specific surface area can be at least 35 m²/g andstill more particularly at least 40 m²/g.

The products in the second implementation of the invention comprisebetween 10% and 20% by weight of titanium oxide. Further, they have aspecific surface area of at least 40 m²/g after calcining for 5 hours at800° C.

They are generally in the form of a mixture of two structures, namely atetragonal ZrO₂ type structure and a monoclinic ZrO₂ structure. Thetetragonal type structure corresponds to a structure with reference79-1771 JCPDS. The monoclinic type structure (baddeleyite) correspondsto a structure with reference 37-1484 JCPDS. The proportions of thesetwo structures vary depending on the titanium oxide content. For thehighest titanium contents, i.e. 20% or very close to 20%, the tetragonalzirconia type structure is in the majority. It is also observed that thestructures are shifted very substantially towards small interplanarspacings, probably due to insertion of Ti⁴⁺ ions into the zirconiastructure. In contrast, for the lowest titanium contents, i.e. closestto 10% and in particular 10%, the monoclinic type zirconia structure isin the majority and it has been observed that this is shiftedsubstantially towards small interplanar spacings, also probably due toinsertion of Ti⁴⁺ ions into the zirconia structure.

The oxides forming the basis of the invention as described above or asobtained from the process described above are in the form of a powder,but they can also be shaped into granules, beads, cylinders orhoneycombs of various dimensions. Said oxides can be applied to anysupport that is routinely used in the catalysis field, such as Al₂O₃ orSiO₂. The oxides can also be used in catalytic systems comprising a washcoat based on said oxides, on a substrate that is, for example, ametallic or ceramic monolith. The coating can also comprise a support ofthe type mentioned above.

Said catalytic systems and more particularly the oxides of the inventionor compositions comprising said oxides may have many applications. Thus,they are particularly adapted to catalyzing a variety of reactions suchas dehydration, hydrosulphurization, hydrodenitration, desulphurization,hydrodesulphurization, dehydrohalogenation, reforming, steam reforming,cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization,disproportionation, oxychlorination, dehydrocyclization of hydrocarbonsor other organic compounds, oxidation and/or reduction reactions, theClaus reaction, the treatment of exhaust gases from internal combustionengines, demetallization, methanation or shift conversion.

In the case of such use in catalysis, the oxides of the invention can beemployed in combination with precious metals. The nature of these metalsand the techniques for incorporating them into said oxides are wellknown to the skilled person. As an example, the metals can be platinum,rhodium, palladium, ruthenium or iridium. In particular, they can beincorporated into the oxides by impregnation.

Of the cited uses, the treatment of exhaust gas from internal combustionengines (automobile post combustion catalysis) constitutes aparticularly advantageous application.

To this end, the invention also concerns the use of a catalyticcomposition comprising an oxide as described above or as obtained by theprocess of the invention to the production of a catalyst for automobilepost combustion.

Some examples will now be given.

EXAMPLE 1

This example concerns the synthesis of a ZrO₂/TiO₂ oxide, 70%/30% byweight.

The starting materials were as follows:

ZrO(NO₃)₂ in solution, with a ZrO₂ concentration of 19.40%;

TiOCl₂ in solution with a TiO₂ concentration of 25.32%;

NH₄OH, 20%.

The first step consisted of preparing 750 ml of a 80 g/I solution ofZrO₂ and TiO₂ oxide. To prepare this solution, 216.49 g of zirconiumoxynitrate solution and 71.09 g of titanium oxychloride solution wereprepared.

The prepared solution was placed in a PROLABO© autoclave and thetemperature was raised to 150° C. The temperature was raised over 75minutes, followed by a constant temperature stage of 6 hours at 150° C.The pressure in the autoclave was also about 4.5 bars. After cooling,ammonia was added to the suspension to produce a pH of 9. The suspensionwas then centrifuged. The cake obtained was taken up in suspension in750 ml of ammonia water (pH=9) then centrifuged again. This operationwas repeated four times. The solid obtained was oven dried for 2 hoursat 110° C. then calcined in a muffle furnace in a confined atmosphere(temperature ramp-up 1° C./min).

The synthesized oxide had a specific surface area of 42 m²/g aftercalcining for 5 h at 800° C. X ray analysis of the product showed thatit was in the form of a pure ZrTiO₄ type structure.

EXAMPLES 2 TO 4

The procedure of Example 1 was followed, but the proportions of thereactants were varied to obtain oxides with different proportions ofzirconium oxide and titanium oxide. The results are shown in thefollowing table.

Proportions, Specific Example ZrO₂/TiO₂ surface area Structures 2 60/4038 m²/g Mainly ZrTiO₄ + anatase TiO₂, shifted very substantially towardslarge interplanar spacings 3 80/20 42 m²/g Mainly tetragonal ZrO₂ withshifts of several peaks + shifted monoclinic ZrO₂, the two structuresare shifted towards small interplanar spacings 4 90/10 40 m²/g Mainlymonoclinic ZrO₂, with certain peaks shifted towards small interplanarspacings + tetragonal ZrO₂

1. A process for preparing an oxide based on zirconium and titanium,comprising the steps of: a) forming a liquid medium containing azirconium compound and a titanium compound, wherein the zirconiumcompound is zirconyl sulphate, zirconyl nitrate or zirconyl chloride andthe titanium compound is titanium oxychloride; b) heating said medium toa temperature of more than 100° C. in order to obtain a precipitate; c)recovering the precipitate obtained at the end of step b); andoptionally, calcining said precipitate.
 2. The process according toclaim 1, wherein said medium is heated to a temperature of between morethan 100° C. and 350° C.
 3. The process according to claim 1, whereinthe reaction medium obtained at the end of the heating step is broughtto a basic pH.